The present invention is related to the seek deceleration operation performed in a disk drive when moving the disk head to a new track, and, more particularly to such an operation wherein the position of the head is sampled at intervals by reading servo data embedded in the tracks as the head passes thereover.
A typical disk drive includes a disk, or platter, on which data is stored using, for example, magnetic or optical storage techniques. A disk head, which reads and possibly also writes data to locations on the disk, is driven by an actuator motor, such as a "voice coil" electrodynamic motor (VCM) having a coil disposed within the magnetic field produced by a permanent magnet. The maximum acceleration and velocity of the head across the disk depends on the strength of the magnets used in the voice coil assembly, the number of windings of the coil and the amount of current flowing through the coil.
The quickest way, ideally, to move a head from one track of a disk to another is to accelerate the actuator as fast as possible for as long as possible, then, at a change-over point, to decelerate as much as possible so that the head stops exactly over the desired track. This is referred to as a "two-bang" acceleration curve, with the first "bang" being a maximum acceleration phase and the second "bang" being a maximum deceleration phase.
A description of the movement of the head across the disk may be thought of as a profile which can be expressed as a family of curves plotting acceleration, velocity or position of the disk head against position or time. The two-bang acceleration curve discussed above is a profile of acceleration versus time. Current, which is related to acceleration, is supplied to the VCM by a controller based on a profile of reference values.
In practice, the profile which would result in the ideally quickest head movement, or "seek", cannot be used, for a variety of reasons. Examples of factors difficult to predict and which affect a seek operation include the individual mechanical characteristics of the disk drive, wear over time which changes the mechanical characteristics of the disk drive, and changing environmental factors, such as a change in the supply voltage available to the motor and a change in temperature surrounding the motor. Consequently, the actual position of the head after following the ideally quickest profile may overshoot or undershoot the desired position.
To avoid overshoot, a disk deceleration operation may attempt to stop the head at a position before the desired track is reached. Since reversing the direction of movement of the disk head consumes substantially more time than moving it in its present direction a few more tracks, it is desirable to define the target stop track as being slightly ahead of the actual desired stop track.
Other techniques have been proposed to minimize the time taken during a seek operation, that is, to attempt to achieve the ideally quickest time in moving the head to a target track. One way to improve a seek operation uses the speed achieved during the acceleration portion of a seek as a factor in determining the reference speed for the deceleration portion; but this method does not address the entire problem.
Another way to improve a seek operation determines the amount of current supplied to the voice coil motor (VCM) based on a signal representing head velocity fed back to the VCM controller from a transducer or, if no transducer is available, from a velocity estimator. This method is most effective when many position samples are available and the disk drive has sufficient processing power. For example, a digital signal processor found in relatively expensive disk drives may support position and velocity sampling rates on the order of several tens of thousands to several hundreds of thousands of samples per second, whereas a microprocessor used in less expensive disk drives may support a sampling rate of only a few thousand samples per second. Thus, a less expensive disk drive may not achieve satisfactory control using only a sensed feedback technique.
A further technique to control the amount of current supplied to the VCM is based upon elaborate calculations performed in response to each position sample. But, less expensive disk drives lack the processing power to perform such elaborate calculations in the interval between samples.
To retain better control over the deceleration, it is known to use an acceleration profile including "ramps", that is, the head is gradually accelerated to its maximum velocity, remains at maximum velocity, then is gradually decelerated. However, due to practical reasons such as those mentioned above, an operating disk drive actuator is difficult to control despite the use of such a "ramped bang" profile.
Another complication arises because the profile is typically stored as a table of values, indexed by distance of the head from the target track. Specifically, this consideration makes it desirable to try to make the actual position samples of the head correspond to distances for which a value is stored, that is, to synchronize the actual samples with those samples having stored reference values. However, storing more values, such as both integral and fractional track positions, corresponding to finer position resolution, requires more memory, and results in a more expensive and larger disk drive.